the turbo buick guys are working on a "stand alone" wide band 02 that might do a bit better. the "narrow band" o2s can be pretty misleading and can be interpreted as a "gross indicator" of WOT a/f.

13.1 is about right for power I would think. maybe just a tad rich, but it's better to be safe. you migh lean it out but only on really good gas. I'll keep you posted on the wide band thing. this could be a really good aid in tuning.